Sizing Check Point appliances for Core Router roles

I have a few clients looking to implement Check Point HA clusters as their core routers with security overlays and am interested in getting your input on how to best assess the existing traffic volume to properly size the appliances for these projects.

Presently, the throughput limitations are the back-plane routing capacity of the L3 switches.

What I am essentially trying to achieve is to quantify combined inter-VLAN routing throughput of the switches, and would like to hear your take on how to achieve that.